Quotes and Analysis from the Star Wars Databank
From the profile of Jedi Master Yoda:
contest between the most powerful practitioners of the Force's light and dark sides.
This line from the Star Wars Databank is in regards to the Duel in the Senate Chamber between the Dark Lord Palpatine and the Jedi Grandmaster Yoda. The line proves that up to that point in time, Sidious and Yoda were the most powerful practitioners of the Force. In the duel, while Sidious bested Yoda by knocking him down the pods, in fact the fight ended in a stalemate with neither side overcoming the other.
Sidious's Force Streght = Yoda's Force Strength
Sidious's Force Strength > Nihilus's Force Strength
Ergo, by the Transitive Property of Equality:
Yoda's Force Strength > Nihilus's Force Strength
With evidence from Lightsnake proving that Yoda had a defense for every Light and Dark Side technique, and Yoda being the most powerful Light side user of the Force up to that point of time and him having a greater Force strength than Nihilus, I'm sure we can conclude that Yoda, in some way possible, can block Nihilus's Ultra Death Field.
More proof on how powerful ROTS Sidious was? Well take a look at this:
From the profile of Palpatine:
He is the most powerful practitioner of the Sith ways in modern times. He studied the ancient ruins on the Sith mausoleum world of Korriban. He unlocked secrets of the Force from a captured Jedi Holocron. The dark side energies flowing through Palpatine's body were so intense, that they ravaged his mortal frame.
Along with other sources, such as the tNEC, etc., Palpatine is the most powerful Sith Lord in existence. Yoda was equivalent to that might. If this is the case, then it seems very very very likely that Yoda can defeat Nihilus.
Want more evidence Nihilus supporters? Well, that'll come soon....
This is General Kenobl signing off.....